A fiber grating is a periodic or aperiodic perturbation of the effective absorption coefficient and/or the effective refractive index of an optical waveguide. It can reflect a predetermined narrow or broad range of wavelengths of light incident on the grating, while passing all other wavelengths of light. Fiber gratings are useful as, for example, filters for wavelength division multiplexing (WDM), gain flattening filters for optical amplifiers, and stabilizers for laser diodes used to pump optical amplifiers.
Typically, fiber gratings are made by laterally exposing the core of a single-mode fiber to a periodic pattern of intense ultraviolet light. The exposure produces an increase in the refractive index of the fiber's core, creating a index modulation according to the exposure pattern. This fixed index modulation is called a grating. At each periodic refraction change, a small amount of light is reflected. All the reflected light signals combine coherently to one large reflection at a particular wavelength when the grating period is approximately half the input light's wavelength. This is referred to as the Bragg condition, and the wavelength at which this reflection occurs is called the Bragg wavelength.
For light signals at wavelengths other than the Bragg wavelength, which are not phase matched, the grating is essentially transparent. Therefore, light propagates through the grating with negligible attenuation or signal variation. Only those wavelengths that satisfy the Bragg condition are affected and strongly back-reflected. The ability to accurately preset and maintain the grating wavelength is a fundamental feature and advantage of fiber Bragg gratings.
As is known, a grating can be produced by using an interferometer to cause two or more optical waves (write beams) to interfere within the core of the fiber, thereby producing an interference pattern therein. The period of a fiber Bragg grating formed by an interferometer can be described by the well-known Bragg equation2nΛ sin θ=mλ  (Eq. 1)where Λ is the grating period, θ is the half-angle between the write beams, λ is the wavelength of the write beams used to form the grating, and n is the index of refraction. The period of a grating need not be uniform. A continuous change in the period of the grating as a function of position along the grating is known as chirp. Chirped gratings reflect different wavelengths at different points along the grating as dictated by Equation 1. As can be seen in this equation, the grating period can be tuned by either varying the write wavelength or the inter-beam angle between the write beams.
In the latter approach, a problem with conventional fabrication methods of fiber Bragg gratings is the inability to change the period of the grating during the fabrication process without changing the position at which the write beams overlap in space or where the fiber is located with respect to these interfering beams. Thus, the need remains for an interferometer which allows for smooth and continuous changes in the period of a fiber Bragg grating during fabrication without repositioning the fiber or the overlap position of the beams.